Many motor vehicles nowadays have a fixedly installed vehicle camera (driver assistance camera), which is usually installed in the upper region of the windshield. The vehicle camera is used in the implementation of various driver assistance systems, which are intended to assist the driver in specific driving situations, e.g. night view assist or lane keep assist system. The vehicle camera can be used, inter alia, to implement distance measurement. Here, active triangulation is used, in which a characteristic pattern is projected by the headlight of the vehicle onto a projection surface (scene), such as a building wall or the road. Lighting the scene with structured light of known optical and/or geometric properties constitutes a core feature of active triangulation. The scene, which includes the projection of the characteristic pattern, is imaged by the vehicle camera. The image of the scene is then subjected to image processing to extract the characteristic light pattern from the camera image. By assigning characteristic features in the projected light pattern to the segments of the headlight producing said features (corresponds to solving the correspondence problem), the distance between the vehicle and the light pattern that is projected onto the projection surface can be ascertained.
In order to implement active triangulation in the vehicle with the required accuracy, a corresponding calibration is necessary. As part of the calibration, the geometric relations between the camera and at least one of the two headlights of the vehicle are determined. The calibration is required for determining the basic length in order to ascertain, based thereon, from the intersection between two beams (a light beam producing the respective characteristic feature in the light pattern and a light beam imaging said characteristic feature in the camera) the distance from said intersection. Here, precise calibration of the stereo system of headlight and camera is necessary to ensure accurate distance ascertainment. If the calibration is imprecise or even incorrect, a corresponding calibration error would have a direct effect on the ascertained distance value, which can have devastating consequences in particular on the road.
The calibration of headlight systems is introduced in accordance with customary methods in connection with a projection of high-resolution patterns (e.g. Gray code pattern). However, such patterns cannot be provided even with current LED headlights (multi pixel headlight systems), which have up to 84 individual LED segments (also referred to as pixels), because the resolution is too low to provide such high-resolution patterns.